Well, I've upgraded one farm to level 4 (which says it produces 1,000 bushels of food per hour) and took over a savannah. Maybe they mean the top right screen where it lists all the resources, that says my production is at 298.

If you're not a ruby player, I'd say about 4 - 6 farms is about right in your capital, but there are many ways of playing this game & I can think of at least 2 players on here who will delight in coming along to tell me I'm wrong! Whatever, you need enough food to be generated until you can attack camps & low level cities, unless you're in an alliance where the overlord is happy to keep you until you can fend for yourself; the quest will kick in at some point during the upgrade of however many farms you choose to build before level 9.

For a more precise answer, see the handbook - the Dragonoconium, or something. It's linked to one of the top messages on this board, I think.